I'd say that's not far from the truth, lots of world leaders knowing as you say, he's only in power for four years so why not placate him.
Another way I look at it, is if he shuts Putin up for four years it's four years he's nearer his grave. In the meantime the EU can rebuild it's defences (yes, I know Putin can to) - but I do feel Trump is right, countries have been taking the piss over their defence spending, if Poland can manage it so can we.
Let's not forget the Tories came in for as lot of stick over their defence spending when they were in power, so again it's narratives that are not far from the truth. We've been happy enough to chuck money down the drain on killing people, money that then has to be spent replacing buildings and peoples arms and legs or burying the dead...
Before Trump came to power:
Some analysts have questioned whether even a rise in defence spending to 2.5% of GDP will be sufficient to combat the rising threats facing the UK. For example, Olivia O’Sullivan, director of the UK in the world programme at Chatham House, contends that to confront Russia and play a leading role in NATO the UK may need to consider moving closer to states like Poland which spend over 3% on defence. She argues that 2.5% is “likely to only fill existing gaps for the current defence equipment plan” (the Ministry of Defence’s 10-year spending plan for equipment and support).
https://lordslibrary.parliament.uk/uk-defence-spending-2-5-of-gdp-target/